
As of writing we have only seen one other mask with its boost and replenish condition (Cat mask, mystic, boosts Willpower/Combat, and replenishes both offerings on doom being placed on a card that doesn't have any).
I think Sparrow Mask will likely be the best, or most universally useful. Willpower and Agility are not the 'do something' stats, when you need to pass a test for these it's often critical for an encounter/scenario card. +2 can push a coin toss to very likely to succeed, and I think this will have wide play for investigators who need it. Bob (especially), Darrell, Rita and Daniella can probably all use this to shore up some of their weaker side (HP for Darrell, Sanity for Rita and Daniella) by shoring up their defenses, while Bob can hand out masks to others.
With respect to the masks in general, they are all (likely) level 0 so they can be used by the Dunwich investigators which also seems strong for Pete (boosting Duke's tests if there is a combat/intellect one).
Edit: How wrong I was, these masks are all insane. Getting primary stat boosts multiple times per scenario is incredibly good, and at only 1 resource even getting the effect twice is fine. Guardians get combat, seekers get Intellect, rogues get agility AND intellect. These are possibly what the talent cards in the base set should have been.